Dimethyl carbonate synthesis from CO2 and methanol over CeO2: elucidating the surface intermediates and oxygen vacancy-assisted reaction mechanism

Surface intermediate species and oxygen vacancy-assisted mechanism over CeO2 catalyst in the direct dimethyl carbonate (DMC) synthesis from carbon dioxide and methanol are suggested by means of transient spectroscopic methodologies in conjunction with multivariate spectral analysis. How the two reactants, i.e. CO2 and methanol, interact with the CeO2 surface and how they form decisive surface intermediates leading to DMC are unraveled by DFT-based molecular dynamics simulation by precise statistical sampling of various configurations of surface states and intermediates. The atomistic simulations and uncovered stability of different intermediate states perfectly explain the unique DMC formation profile experimentally observed upon transient operations, strongly supporting the proposed oxygen vacancy-assisted reaction mechanism.


Introduction
2][3][4][5][6][7] While one of the major synthesis paths of DMC is via the reaction of methanol with phosgene (COCl 2 ), nowadays the industrially recognized processes involve either oxidative carbonylation reactions using CO (Enichem, Ube and Bayer AG), trans-esterication reactions of organic carbonates with methanol (Asahi Kasei), or a two-step urea methanolysis method developed by Catalytic Distillation Technologies, Inc. 8 Despite these improvements, it can be easily understood why a safer, more eco-efficient and economic pathway for DMC synthesis is desired.The development of the direct reaction between CO 2 and short chain alcohols, particularly methanol (MeOH) and ethanol (EtOH), for the synthesis of organic carbonates such as DMC or diethyl carbonate (DEC) (Scheme 1, DMC synthesis) has drawn great attention in the light of urged CO 2 chemical xation, green chemistry (high atom efficiency processes), and safety by replacing the highly toxic and corrosive phosgene molecule by CO 2 as carbonyl source.
In the last 20 years, various homogeneous and heterogeneous catalysts were reported for the direct carboxylation reaction of methanol using CO 2 .Heterogeneous catalysts are more widely investigated and different materials have been tested under both batch and continuous operation.Among them, zirconia (ZrO 2 ), ceria (CeO 2 ), and ZrO 2 -CeO 2 solid solutions [9][10][11] have been reported as the most effective catalysts in the direct

EDGE ARTICLE
DMC synthesis, while other catalyst materials afforded no or very little formation of DMC. 12 Generally, the presence of both acidic and basic sites is required for the activation of CO 2 and methanol.Indeed, the aforementioned reported active catalysts possess both the acidic and basic properties. 13Nevertheless, in practice the reaction is highly thermodynamically limited and the DMC yield can be enhanced only up to about 1% even under thermodynamically favourable high-pressure conditions (ca.400 bar). 14Recently, a drastic improvement in DMC yield (>90%) was reported by Tomishige et al. over CeO 2 as the unique catalyst through in situ dehydration where 2-cyanopyridine (2-CP) serves as organic dehydrating agent to remove the water byproduct (Scheme 1).CeO 2 functions as catalyst in both DMC synthesis and dehydration reaction optimally at ca. 120 °C.5][16][17] It is noteworthy that the rate determining step of the DMC synthesis is unaltered by the presence of 2-CP.This implies that understanding the simpler reaction system without the dehydrating agent would be of direct relevance in understanding more complex yet more practical system with the dehydrating agent. 16he unique catalytic performance of CeO 2 in this reaction may originate from the synergy between its acid-base and redox properties.Izumi et al. studied the reducibility of CeO 2 promoted with Cu (0.1 to 0.5 wt%) by X-ray absorption spectroscopy (XAS) aer high temperature reduction treatment (400 °C) and subsequent CO 2 adsorption.They suggested that the partial reduction of Ce 4+ sites to Ce 3+ may be benecial for DMC formation. 18Also, Aresta et al. correlated by ex situ X-ray photoelectron spectroscopy (XPS) the origin of catalyst deactivation, thus the activity, with the oxidation state of Ce. 19,20 In a more recent study, Li et al. synthesized Zr-doped CeO 2 nanorods, and they linked the DMC production rate to the highest number of oxygen vacancies (i.e.extracted from ex situ Raman and XPS measurements; versus bare CeO 2 ) that allows an increased CO 2 adsorption rate. 21Despite all these indications, the contribution and importance of the redox properties or surface defects of CeO 2 in the reaction has not been proven under working (operando) reaction conditions.Particularly, the reaction is commonly performed at relatively low temperatures (100-150 °C) where the reduction of surface and bulk CeO 2 is unlikely to occur according to temperature programmed reduction studies [22][23][24] and the detection of electronic structure change of Ce, if any, is expected to be challenging.Furthermore, no operando spectroscopic investigation has been reported bridging the type of surface species present under reaction conditions with DMC formation by simultaneous detection of the product concentration.
Following this background, in this work we elucidate the surface chemical intermediate, at its energetically resting state, leading to DMC formation by means of operando diffuse reectance infrared Fourier transform spectroscopy (DRIFTS) coupled with multivariate spectral analysis [25][26][27] which enables unreferenced spectral separations (ESI, Fig. S1 †).Furthermore, through density functional theory (DFT) calculations, XAS and Raman spectroscopy, the critical roles of surface oxygen vacancies of CeO 2 at the rate limiting step are uniquely unravelled.

Results and discussions
Fig. 1 presents time-resolved operando DRIFT spectra and DMC concentration prole under a periodic concentration change of methanol (16.5 vol% in He, the rst half period of 128 s) and CO 2 (the second half period of 128 s) performed at 120 °C (averaged over 8 periods to improve S/N).The MS signal (Fig. 1, le) shows that DMC was formed under the studied condition.The production level of DMC increased upon switching from CO 2 to methanol, reaching a constant level aer ca.40 s.Interestingly, the DMC production was boosted by ca. 4 times upon switching from methanol to CO 2 and then gradually ceased with time, almost completely under the ow of CO 2 .The stable activity aer 40 s is at the same level as that of the steadystate activity of DMC formation by passing MeOH + CO 2 (ESI, Fig. S3 †).This means that MeOH vs. CO 2 perturbation creates favourable condition for DMC formation, enhancing the formation rate by 4 times compared to the steady-state activity.The differential DRIFT spectra in the low frequency region from 950 to 1800 cm −1 (Fig. 1, right; the last spectrum of the period, i.e., at the end of the CO 2 period, was used as the background) shows the characteristic C-O stretching vibrations of bridged and terminal methoxy species appearing at ca. 1060 and 1120 cm −1 , respectively, 10 as positive bands (in red) when the atmosphere was switched to methanol.Likewise, the absorbance of the methoxy bands (hence the concentration of methoxy species) for the MeOH + CO 2 vs. CO 2 experiment is ca. 4 time less compared to the case of MeOH vs. CO 2 , well in-line with the semi-quantitative analysis of the MS proles and the observation from Fig. 1, le which tells us that methanol adsorption needs to take place rst in order to activate the DMC formation (ESI, Fig. S4 † for more detailed discussion).
At the same time, several negative bands characteristic of carbonates/bicarbonates were observed in the region from 1150-1800 cm −1 under the ow of MeOH.9][30][31] The results clearly indicate that carbonates/bicarbonates formed under CO 2 atmosphere were replaced (thus the bands appear negative) by the methoxy species under methanol atmosphere, and vice versa under CO 2 atmosphere.At rst glance, the DRIFT spectra do not show signatures of transient surface species whose concentration is similar to that of DMC, except a few slightly positive signals at ca. 1310 and 1630 cm −1 appearing upon switching to CO 2 .This sort of ambiguousness in spectral analysis is oen encountered and arises from overlapping peaks and also impossibility to deconvolute the spectral contributions of different chemical species due to the unavailability of proper reference spectra of the chemical species that are only present/detected under reaction conditions and/or under transient conditions.
To overcome this limitation, multivariate spectral analysis was employed to disentangle overlapping peaks into the spectra of "kinetically pure" components (i.e., surface species).Besides the spectral separation, it yields conveniently the corresponding concentration proles of the components.Provided that the data quality is good in terms of S/N, the multivariate spectral analysis can overpower another famous technique used for detection of minor species, i.e., modulation excitation spectroscopy (MES, using phase sensitive detection), especially when there is a high degree of spectral overlap and species with extremely different kinetics are involved.3][34] Fig. 2 shows the results of the multivariate spectral analysis on the DRIFT spectra shown in Fig. 1.For comparison, the results obtained via MES analysis are included in the ESI, Fig. S5.† The analysis identied three kinetically distinct components.Most strikingly, the analysis could separate the spectral component (Fig. 2, green) with the concentration prole perfectly matching with that of DMC (Fig. 1, le).The other two components show characteristic features of (i) methoxy species (Fig. 2, black) and (ii) carbonate/bicarbonate species (Fig. 2, red).The corresponding concentration proles of these components (Fig. 2, right) obviously reect the atmosphere of the gas phase.
6][37] MMC further reacts with methanol (methoxy) to afford DMC.Later, it was indicated that the reaction proceeds via the same intermediate (i.e.MMC) over CeO 2 catalysts. 10,16There are also reports supporting the existence of a different intermediate like carbomethoxide (CH 3 OCO-Ce) species as suggested by Wang et al. 28 The spectral characteristics of the captured intermediate (Fig. 2, green) whose concentration prole resembles that of gas phase DMC generally show the mixed features of methoxy (1060 and 1120 cm −1 ) and (bi)carbonates (e.g.1200-1800 cm −1 ), although they are somehow different especially in the region of 1200-1600 cm −1 where the characteristic bands of MMC are expected to appear according to the literature. 10,20,28,38More precisely, Lavalley et al. assigned the two bands we observed at ca. 1330 and 1460 cm −1 to a coupling between the bending mode of CH 3 and the stretching vibration of OCO. 39This clearly indicates that the captured intermediate is MMC or species alike.The spectral features of the intermediate (Fig. 2, green) in the methoxy region appear similarly to those of methoxy species (Fig. 2, black).A closer look veries that they are similar but there are noticeable differences especially in the band of terminal methoxy at ca. 1120 cm −1 of the intermediate, showing a broadening and a small shi towards lower vibrational frequency (ESI, Fig. S2 †).This implies that the terminal methoxy has reacted or is interacting with the CO 2 molecule trapped on the surface, resulting in the red shi.Also, intriguingly the species responding to the gas-phase methanol concentration (Fig. 2, black) show the spectral features of (bi) carbonates in the region 1200-1800 cm −1 .This is indicative of the formational correlation of methoxy species with specic surface (bi)carbonate species.The dissociation of MeOH into MeO and H is known 10 to take place upon methanol adsorption over CeO 2 and this surface adsorbed H may lead to more pronounced formation of bicarbonates from carbonates (see the discussion below for DFT calculations) and the spectral feature of the methoxy species may reect the accompanied formation of bicarbonate surface species since the multivariate spectral analysis cannot disentangle spectral components behaving kinetically identical.A similar discussion for the high frequency region (above ca.2000 cm −1 ) is presented in the ESI (Fig. S6 and S7 †) including the difference DRIFT spectra and the corresponding multivariate spectral analysis results.
Under similar transient conditions we looked into the change in the oxidation state of Ce by XAS at Ce K and L3-edges and structural changes by Raman spectroscopy.Under oxidation (O 2 ) and reduction (H 2 ) treatment at high temperature (350 °C), the multivariate spectral analysis on the XAS and Raman data could clearly extract the redox features of CeO 2 (ESI, Fig. S8-S10 †), whereas these redox features could not be observed at the reaction temperature of 100-150 °C which was obviously too low for the redox to take place as expected.Although we could not detect spectral changes in the bulksensitive XAS under the transient reaction conditions of Fig. 2 (left) Three components spectra and (right) the corresponding concentration profiles obtained by multivariate spectral analysis applied on the DRIFT spectra shown in Fig. 1 (the first half is methanol and the second half is CO 2 atmosphere).To ease the data analysis and improve component separation, the spectral analysis has been performed on two separate regions of methoxy (below 1170 cm −1 ) and carbonates (above 1180 cm −1 ).Both spectra and concentration profiles are normalized for clarity.
MeOH vs. CO 2 ow, there was a clear change in the F 2g Raman active mode at ca. 460 cm −1 , corresponding to the symmetric breathing of O 2− atoms vibrations around the Ce 4+ cations (Fig. 3).The width and position of this band is known to be extremely sensitive to any structural disorder of the O-sublattice. 40Multivariate spectral analysis on the Raman data disentangled the two spectral components with contrasting concentration proles: one increasing upon CO 2 exposure which is obviously assigned to the band associated with the Ce 4+ state (Fig. 3, Component 1, red), and the other increasing with methanol with the band feature slightly red-shied and broadened (Fig. 3, Component 2, black).2][43] Yoshimura et al. use the changes in Raman-allowed F 2g mode (i.e.shis to lower energies, and the line shape getting progressively asymmetric with the crystal size decrease) in a study about the identication of defects in ceria-based nanocrystals by UV resonance Raman spectroscopy. 44This subtle change took place on the surface of the material and therefore could not be measured by XAS.Likewise, the change was less obvious when a longer wavelength excitation laser (785 nm) was used in Raman since in this case we sample more bulk of the material (ESI, Fig. S11 †).Given the magnitude of the spectral change and the mild reaction conditions it becomes rather difficult to state the existence of a complete Ce 4+ 5 Ce 3+ cycle despite a recent publication by Tomishige et al. which reported the redox properties of CeO 2 catalysts in organic reactions (i.e.[47][48] Based on what has been reported, the creation of a nonstoichiometric CeO 2−d surface via the formation of some surface and subsurface O-vacancies (i.e.defective surface) is possible and explains the Raman results.Such defective surface formation would lead to a more electron decient or more electron rich CeO 2 state depending on the gas atmosphere.Glatzel et al. have previously invoked this picture of an electron sponge for CeO 2 nanoparticles studied by HERFD-XAS. 49They found an increase of the interatomic distances between Ce and O during the catalytic decomposition of hydrogen peroxide while stating that the redox partner is not a local Ce 3+ site, but the electron density that is received and released during the reaction is delocalized over the atoms of the nanoparticles.Furthermore, the emergence of two bands in the 2000-2250 cm −1 region of the DRIFT spectra that can be linked to Ce 3+ surface sites formation induced by methanol adsorption over CeO 2 can further support the Raman results and thus methanol adsorption-induced Ce 3+ formation (ESI, Fig. S6 †) which is important for the enhanced DMC formation (Fig. 1, le, Fig. S3 †).
The mechanistic investigation by DFT calculations strongly supports our experimental observations and claries atomistic and electronic insights.The computational details are described in ESI.† While methanol adsorption occurs preferentially via the dissociation of the molecule into CH 3 O − and H + , the favoured and most stable adsorption state of CO 2 is represented by a monodentate carbonate species involving a covalent bond between the C atom of the CO 2 molecule and surface O atoms belonging to CeO 2 (see ESI, Fig. S12 † for adsorption structures).
Here, we considered the two reaction pathways for DMC formation, one starting from an adsorbed CO 2 molecule that is a monodentate carbonate species, CO  is inserted into the adsorbed methoxy species, leading to the formation of monomethyl carbonate (MMC) species.This MMC species is nucleophilically attacked by the surface oxygen atom, which results in the formation of MMC S by accepting a proton from nearby methanol molecule.The subsequent reaction pathway from MMC S is the same as that in Path A. Fig. 5 shows the free energy proles along the reaction pathways.In the rst step of Path A, the CO 2 molecule is adsorbed on the CeO 2 surface with a small barrier of 21 kJ mol −1 (CO 2 * / CO 2 (m)), forming a monodentate structure CO 2 (m).Then, the surface monomethyl carbonate species (MMC S ) is generated with an activation energy of 77 kJ mol −1 (CO 2 (m) / MMC S ).Before reaching to this transition state congurations, we nd that a proton is transferred from a neighbouring methanol molecule to the surface carbonate species, resulting in the bicarbonate species.This bicarbonate species is attacked by methoxy moiety to form MMC S structure (see the inset of CO 2 (m) / MMC S in Fig. 5 and the snapshot near the transition state structure is shown in ESI, Fig. S13 †).The resulting MMC S structure is less stable, and the carbomethoxy species (INT_1.1) is formed with a small barrier of 13 kJ mol −1 by eliminating a water molecule (snapshot near the transition state structure is also provided in ESI, Fig. S13 †).This carbomethoxy conguration is meta-stable and it readily changes its conguration with a more stable monomethyl carbonate state accompanied by the creation of an oxygen vacancy during the MD simulation (shown as INT_1.2, and snapshot is provided in ESI, Fig. S13 †).This intermediate is further attacked by another methoxy moiety, leading to the formation of INT_2 with an activation barrier of 49 kJ mol −1 .During this attack of methoxy species, the surface vacancy is relled by the oxygen atom.The nal step is a bond cleavage between the carbon and surface oxygen atom, and this step requires an activation energy of 52 kJ mol −1 .The free energy difference between INT_1 and the transition state between INT_2 and DMC* is estimated to be 87 kJ mol −1 , which is comparable to the experimental value of 73 kJ mol −1 reported in ref. 16.
Next, the reaction mechanism of Path B is analysed in detail.In the rst step, the monomethyl carbonate species is formed by the reaction between a dissociated methanol and CO 2 with an activation barrier of 37 kJ mol −1 (CH 3 O − * / MMC).The conformation of MMC species during the MD simulation is close to the adsorption structure of DM1 or DM2 shown in ESI, Fig. S12, † where in the MD simulations the hydrogen bond of the carbonyl group is observed with hydrogen in the methanol molecule (snapshot is provided in ESI, Fig. S13 †).The next step is a nucleophilic attack of the surface oxygen atom to the carbon atom accompanying a proton transfer to the carboxylate group (MMC / MMC S ).This process requires activation energy of 82 kJ mol −1 .
The gaseous product (DMC) prole (Fig. 1) is in good agreement with the mechanistic insights obtained by the DFT calculations.Upon switching from CO 2 to methanol, there is an induction period before reaching a stable DMC production.This clearly indicates a strong CO 2 adsorption to the surface sites; it takes some time to replace the strongly adsorbed CO 2 molecules by methanol.This competitive adsorption and replacement by methoxy retards the DMC formation and also implies that the methoxy and Ce 3+ generation are crucial for the reactivity towards DMC formation.Also, the spectral features of bicarbonates in the region 1200-1800 cm −1 in the methanol atmosphere (Fig. 2, black) correspond to the bicarbonate species before the formation of MMC S structure.On the other hand, upon switching from methanol to CO 2 there is an immediate formation of DMC with a boost in productivity, demonstrating that for the CO 2 molecules it is much easier to kick out the previously adsorbed methanol from the surface through MMC intermediate.Aer that, DMC production ceases quite fast suggesting a very strong CO 2 absorption to CeO 2 surface and the lack of any methoxy species in the proximity.Based on the discussion above, we can also conclude that the stability of the intermediate (INT_1.2) and the creation of a defective surface is the key to make CeO 2 an efficient catalyst for the direct synthesis of DMC while the reaction invokes the image of a Langmuir-Hinshelwood type of mechanism with methanol and CO 2 molecules adsorbed in the immediate vicinity of a Ce-O pair.

Conclusions
In summary, the present study describes the in situ creation of a defective CeO 2 surface and its involvement in the gas phase DMC synthesis from CO 2 and methanol by using a combination of experimental and theoretical tools.While methanol seems to increase the electron density around Ce sites, CO 2 behaves as an oxidizing agent leading to fully oxidized Ce 4+ .Furthermore, a boost in DMC production is observed when CO 2 is passed over methoxy covered surface.The surface intermediate species directly correlated with DMC formation was elucidated by operando DRIFTS with the aid of the multivariate spectral analysis.It shows the feature of both methoxy and (bi) carbonates, in good agreement with the reported structure of monodentate methyl carbonate.We demonstrate, for the rst time ever, the reconguration of this species on the CeO 2 surface via a vacancy-assisted mechanism, observation which correlates well with the operando Raman results.
Ultimately, direct DMC synthesis from methanol and CO 2 is un-equivocally driven forward by the presence of neighbouring acid-base pairs over CeO 2 surface.Nevertheless, the inuence of O-vacancies in the reaction mechanism and the extraordinary redox (surface) properties of this oxide cannot be neglected anymore.It has already been shown that an increased surface reducibility which can ease the formation of surface defect sites responsible for the intermediate stabilizationplays critical roles in the overall reaction mechanism involving the in situ water removal by 2-CP molecules (while the DMC formation mechanism is unchanged by the dopants and mainly catalysed by CeO 2 surface). 50Hence, this study establishes new directions for designing better and more stable catalysts for this newly implemented and highly active route to DMC synthesis by direct methanol carboxylation reaction.

Scheme 1
Scheme 1 Direct DMC synthesis from CO 2 and methanol.

Fig. 1 (
Fig. 1 (left) MS signal of DMC (m/z = 59) and (right) time-resolved DRIFT spectra taken during methanol (the first half period) vs. CO 2 (the second half period) concentration perturbation experiment performed at 120 °C at 7 ml min −1 .The time-axes (y-axis) of both figures are identical.The DRIFT spectra were calculated taking the last spectrum in the CO 2 atmosphere as background.
In literature, Mullins et al. probed the surface sites of CeO 2 nanocrystals with well-dene surface planes via methanol adsorption, and they observed slight reduction of the rods (110) and cubes (100) surfaces at room temperature by a UV Raman study.In addition, surface science reports on methanol adsorption over thin-lm model CeO 2 catalyst support the reduction phenomenon occurring in the topmost oxide layers even at ambient or sub-ambient temperatures (Mullins et al. and Skala et al.).

Fig. 3
Fig.3Multivariate spectral analysis applied for F 2g band of CeO 2 during DMC synthesis experiment (CO 2 vs. methanol) at 140 °C.The Raman measurement was performed with a 532 nm laser (green).Both spectra (left) and concentration (right) profiles are normalized for clarity.

Fig. 4
Fig. 4 Reaction mechanism of DMC formation from CO 2 and methanol.Path A starts with an adsorbed CO 2 molecule while Path B starts from the dissociative adsorption of a methanol molecule.

Fig. 5
Fig. 5 Free energy profiles along the two reaction pathways.The free energy of the monodentate structure CO 2 (m) is set to zero, and relative energy is shown in parenthesis.Activation energy (DA in kJ mol −1 ) in each step is also included.The inset on the bottom (INT_1 = INT_1.1 / INT_1.2) evidences the key step in the stabilization of the reaction intermediate and formation of an O-vacancy in the catalytic cycle over CeO 2 .